How do cyclones affect the Great Barrier Reef?

The Bureau of Meteorology’s latest Severe Weather Forecast predicts we’ll experience a below-average number of tropical cyclones this storm season, which runs from November 2023 to April 2024.

It says the eastern region, which includes the Great Barrier Reef, typically experiences four tropical cyclones each storm season, but El Niño historically reduces the total number of tropical cyclones which form in the Australian region.

In El Niño years like this one, the first cyclone to make landfall on the Australian coast typically occurs later than normal, around the middle of January. However, the early arrival of Cyclone Jasper in Queensland waters is an anomaly as the first tropical cyclone to form in December under these climate conditions.

Top image: Thomas Horig, Ocean Image Bank

All three terms refer to tropical storms and the only difference between them is their location. They’re known as cyclones when they form over the South Pacific and Indian Ocean, and hurricanes when they occur over the North Atlantic, central North Pacific and eastern North Pacific. Tropical storms that form in the Northwest Pacific are called typhoons.

A tropical storm is declared and given a name when wind speeds consistently reach 63kph. Their intensity is rated on a five-point scale, with category five being the strongest. 

Cyclone Debbie, which made landfall in 2017 near Airlie Beach, was a category four storm that caused widespread damage across the Whitsundays. Wind gusts reached 263 km/h.

These strong winds whip up very large, powerful waves and when they break over coral reefs, they can tear corals from their footing and turn large sections of the reef to rubble. Research by the Australian Institute of Marine Science (AIMS) shows cyclones can cause damage up to 1,000km away from their path.

Climate change is causing higher temperatures on land and in our oceans, which is increasing the frequency and intensity of tropical storms around the world. Coral reefs have the ability to recover naturally from the impacts of cyclones, hurricane and typhoons, but it can take a decade or more. When tropical storms occur in quick succession, there often isn’t enough time for reefs to recover before they’re battered by the next one.

Cyclones leave behind rubble beds of dead coral skeletons and rock fragments that constantly move with the tides and waves. It’s much harder for baby corals to settle and grow on unstable and moving surfaces and corals that do manage to settle can be killed as they tumble around. This makes it much harder for the Reef to naturally recover following a cyclone.

Over time, rubble beds eventually stabilise with the help of natural binders like encrusting algae and sponges to form a stable platform for new corals. But with increased bleaching events and storms the amount of rubble on the Reef is rising.

Where unstable rubble restricts new coral growth, we have two ways to promote reef recovery:

1. Rubble stabilisation

We use artificial tools like mesh netting and carbonate rocks to stabilise rubble so new corals can grow. We also replace damaged or lost reef with new structures, including specially-designed 3D shapes.

2. Planting corals

We capture coral spawn and rear baby corals in floating pools on the Reef. When they are ready, we deliver the babies to damaged areas to help repopulate them. We call this technique, Coral IVF.

We also collect broken pieces of coral from the sea floor and grow them in underwater nurseries. When they’re ready, we plant them on the Reef.